1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filters having an improved transmission characteristic suitable for band-pass filters of communication apparatuses, such as portable phones, and to communication apparatuses including the same.
2. Description of the Related Art
In compact communication apparatuses such as portable phones, band-pass filters having a pass-band of several tens of MHz to several GHz have been commonly used. As an example of the band-pass filters, compact SAW filters have been used.
As shown in FIG. 25, a SAW filter 500 includes a filter element 504 having a reflector 510, interdigital transducers (IDTs) 501 to 503, and a reflector 511, which are arranged along a SAW-propagating direction on a piezoelectric substrate 100. Herein, each of the IDTs 501 to 503 provides an electric signal to a SAW coupling transducer including a pair of comb electrodes engaged with each other.
An input pad 520, an output pad 521, and ground pads 522 to 524 are disposed on the piezoelectric substrate 100, and wiring traces 525 to 530 for electrically connecting the IDTs 501 to 503 and the pads 520 to 524 are also disposed on the piezoelectric substrate 100.
All of the IDTs 501 to 503, the reflectors 510 and 511, the pads 520 to 524, and the wiring traces 525 to 530 are portions of a conductive thin-film pattern on the piezoelectric substrate 100.
When an electric signal is applied to the input pad 520 of the SAW filter 500, a surface acoustic wave (SAW) is excited by the IDTs 501 and 503, and a standing wave of the SAW is generated in a region including the IDTs 501 to 503 sandwiched by the reflectors 510 and 511. Then, the IDT 502 transforms the energy of the standing wave to an electric signal, such that an output potential is generated at the output pad 521. A transform characteristic of transforming an electric signal to a SAW by each of the IDTs 501 to 503 has a frequency characteristic, and thus, the SAW filter 500 has a band-pass characteristic.
The SAW filter 500 shown in FIG. 25 is a longitudinally coupled resonator SAW filter, in which the IDTs 501 and 503 for input and the IDT 502 for output are acoustically cascaded in an acoustic track sandwiched by the reflectors 510 and 511. In place of the longitudinally coupled resonator SAW filter, a transversely coupled resonator SAW filter, a transversal SAW filter, a ladder SAW filter, and a lattice SAW filter may be used.
In any type of SAW filters, a conductive thin-film pattern defining IDTs and wiring traces are disposed on a piezoelectric substrate, and a band-pass characteristic is obtained by using a frequency characteristic of an electric signal to SAW transform function of the IDTs.
Also, in known arts, at least portions of wiring traces are three-dimensionally crossed with each other such that an insulator including SiO2 or other suitable material is provided therebetween, so as to miniaturize SAW filters (see Patent Documents 1 to 5):
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-167387 (Publication Date: Jul. 2, 1993);
Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-235684 (Publication Date: Sep. 10, 1993);
Patent Document 3: Japanese Unexamined Patent Application Publication No. 7-30362 (Publication Date: Jan. 31, 1995);
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-49567 (Publication Date: Feb. 18, 2000); and
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-138553 (Publication Date: May 16, 2000).
In the known SAW filters, a filter characteristic is deteriorated by parasitic capacitance generated between wiring traces on a piezoelectric substrate. Parasitic capacitance generated between a wiring trace receiving an input signal and a wiring trace generating an output signal serves as a current bypass from an input-signal terminal to an output-signal terminal. Therefore, the parasitic capacitance degrades the suppression level for signals of frequencies outside a pass band.
In particular, a SAW filter having many IDTs requires many wiring traces for connecting the IDTs. Further, the covered area is increased, parasitic capacitance is more likely to be generated, and the size of the filter is increased.
In a SAW filter having a balance-to-unbalance transformer function, in which one of input and output is an unbalanced signal and the other is a balanced signal, parasitic capacitance between a wiring trace receiving an unbalanced signal and a wiring trace receiving a balanced signal serves as a current path for bringing unbalanced signals of same phase and same amplitude to two balanced signals which usually must have opposite phases and the same amplitude. Therefore, a common-mode signal in each balanced signal increases, such that the degree of balance is deteriorated.
As described above, parasitic capacitance between wiring traces, in particular, parasitic capacitance between wiring traces having different potentials, has a detrimental effect on the characteristic of a SAW filter. Specifically, when a piezoelectric substrate includes a material having a relative permittivity of more than about 20, for example, LiTaO3, LiNbO3, or Li2B4O7, parasitic capacitance substantially increases, and thus, the detrimental effects are significant. Also, larger current flows through the parasitic capacitance as the frequency increases. Therefore, a SAW filter having a higher-frequency pass band is more severely affected.